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Food writer Kathy Paterson doesn't need to think about which shoes to wear when she gets dressed in the morning. For the past year or more she has worn her "uniform" almost every day: casual wool shoes by online company Allbirds.

Paterson is an evangelist for the unusual sneakers, dubbed "the world's most comfortable shoes" by Time magazine.

She has converted many others to wearing the New Zealand merino wool shoes, she reckons, and at Christmas she bought them as gifts for her parents and sister.

Paterson has two pairs in rotation. "They're incredibly comfortable," she says. "I do not take them off, winter and summer.

Prime Minister Jacinda Ardern can regularly be spotted wearing them; she gave pairs as gifts to Australian PM Malcolm Turnbull and his wife on her recent visit to Australia. Chatshow queen Oprah Winfrey has also been given a pair.

Jacinda Ardern is a fan of Allbirds shoes.

The Silicon Valley tech sector has also adopted Allbirds as its shoe uniform. The New York Times ran a story last August headlined: "To Fit into Silicon Valley, Wear These Wool Shoes", and reported top tech company heads and venture capitalists as fans. Silicon Valley, it said, "likes a uniform. Standing out with a personal style in tech is generally shunned, since it implies time spent on aesthetic pleasures, rather than work."

The simplicity of Allbirds fits that thinking. The shoes are unadorned with logos or extraneous detail, come in muted colours and two styles only — a sneaker and a loafer, selling for $160.

It's a simplicity Allbirds co-founder Tim Brown calls "the right amount of nothing. It's this pared-back execution that is shocking for its honesty, in a way".

Brown's quest for simplicity in a shoe goes back 15 years, to his days as a professional football player. The 37-year old played for Wellington Phoenix and was a member of the successful All Whites squad that made the World Cup finals in 2010.

"I was leading this very fortunate existence doing something I'd tried to do for a long time", he says. "I was sponsored by a big sportswear company and got lots of free gear."

The free gear wasn't what he wanted to wear, though.

"It was very, very hard to find footwear that didn't have logos all over it. Everything was brightly coloured, loud, over-designed. And I had to wear a lot of it.

In his off-season, as a side project, Brown travelled, found a footwear factory and started making some shoes.

"I started selling them to my teammates, who told me they were rubbish. But I kept on going anyway."

Brown's idea his shoes could be made of wool, something that hadn't been done before, sent him "down the rabbit hole" of the project, he says.

A research grant allowed for the development of fabric strong enough to make shoes, a process that took so long he "almost forgot about it".

His playing days over, in 2012 Brown found himself at business school in London with "a little bit of shoe making knowledge and this first generation fabric. I had half an idea and no real sense of what I was going to do next".

A push from one professor, Carter Cast — a former Walmart.com CEO who later became an Allbirds investor — made Brown think of his shoes as a business, albeit reluctantly.

"We kind of came to the conclusion that it was a bad category, that shoes were too hard to make, and that wool shoes probably wouldn't work. But for whatever reason I should try it. "[Cast] said I should take this idea to the world and work out whether it was a good or bad one."

Brown did that in the form of a 2014 Kickstarter campaign, with a video shot on a friend's farm north of Wellington. The response answered the question of whether the shoes were a good or bad idea.

"I remember sitting in a restaurant in London. For four days my phone had buzzed every time there was an order. We sold $120,000 worth of shoes in four days.

"I had to stop the whole thing because I didn't have enough of the wool to make any more shoes than that. It was one of those moments when you realise, gosh, this story has resonated for whatever reason."

After an "incredibly hard" first year, Brown had what he describes as "one of those perfect meetings of the mind" with Joey Zwillinger, a San Francisco-based engineer and renewables expert. The two decided to join forces on Allbirds.

It was Zwillinger who got Brown focused on — and fired up about — sustainability.

"Joey helped me realise we had an ideal opportunity to be part of a revolution in the way things were made. Wool was a renewable resource, an incredibly sustainable fibre when farmed in the right way.

"There was a real opportunity here to make products that weren't just comfortable and well designed but that were also better for the environment. That was a huge unlock for me."

From that moment, Brown says, he hasn't looked back.

"It was when I realised there was a much larger purpose here over and above just making a great product. I think I've always had empathy for the environment and this movement. I don't think I ever thought I'd be in a position to help it."

Footwear is an industry not known for being kind to the environment. A review published last year by lobby group Change Your Shoes reported over 22 million pairs of shoes are produced each year, and noted "behind our shoes ... there can be human rights violations or harming of the environment in many places along the supply chain".

"The footwear industry is incredibly antiquated. The first time I went to a footwear factory I couldn't believe how labour intensive the process was and I had this glaring observation that everything was done cheaply and made out of pretty horrible materials."

The footwear industry has "got lazy", he says, when it comes to materials and paid lip service to the idea of sustainability.

By the time Allbirds relaunched in March 2016, selling one product only online, Brown had moved to San Francisco, "raised some money" (an understatement: Allbirds raised more than US$2.7m ($3.7m) in venture capital, followed by a further $9.9m in September 2016) and sustainability was a big part of the brand's story.

Apart from the sustainably grown wool, which the company says requires 60 per cent less energy to produce than typical synthetic materials, Allbirds shoes' insoles are made from castor-bean oil. The Allbirds shoe box uses 40 per cent less materials than traditional packaging.

The company was formed as a benefit corporation — the environment is one of the key stakeholders in its governance structure.

It is a certified "B Corp" — the equivalent of Fair Trade certification for coffee. And it has recently had its first life cycle analysis to audit the carbon output of every aspect of its supply chain.

The Allbirds Wool Runner in Natural White. Photo / Vaughan Brookfield

"There are so few examples of companies that have done that in footwear," says Brown.

Still, it has to be tricky to balance sustainability with the distances materials and shoes have to travel. Wool from New Zealand travels to be milled into fabric in Italy, then heads to South Korea to be made into shoes before they head to the US.

Allbirds sells direct to customers online and shoes are shipped around the world.

Although he is not prepared to release details of the recent audit, Brown admits achieving perfect sustainability is an ongoing journey.

"Sustainability is not a destination, it's a mindset," he says.

"And achieving that is something you will never ever do because it's so hard and you really need to focus on the small steps towards that goal."

Can Allbirds maintain its dedication to sustainability and keep growing?

Brown is adamant it can be done and cites a recent example of the philosophy in practice.

His team had worked out a way to use post-consumer material, from plastic bottles, to make shoelaces, and met to discuss making the change.

"It was a tripling in cost for that item, which effectively performs exactly the same, that looks exactly the same, and no one would know [had changed].

"And what was incredible was that we made that decision in 10 seconds. There were people in that meeting with competing priorities who were charged with improving our margins and improving our efficiency overall financially who straight away knew that was the right thing to do.

Shepherding merino at Northburn Station. Photo / Vaughan Brookfield

"We have built this thing with that mindset embedded in our DNA. It's not a nice to have, it's a non-negotiable. I think when you operate in that way it's a bit easier."

Brown hesitates before talking about the million-shoe milestone.

"It's pretty amazing to say that out loud," he says.

"We've been really careful to stay focused on what we're really all about and not get too caught up with numbers and financial performance. But it's a really significant milestone."

There's pride, too, when he talks of the New Zealand connection.

"I think as a Kiwi what has been amazing for me on this journey is to be so connected back to New Zealand with the evolution of this business and the use of this fibre."

Wool, he says, is having "an incredible moment. Being a part of telling this story is pretty awesome."

He recently brought some of his 100-strong team to Invercargill for the world sheep-shearing championships.

"It was amazing meeting these incredibly passionate people," he says.

"For the longest time, I think, no one had been growing up dreaming of being a sheep farmer, and I think that's changing."

The next chapter for Allbirds is not about wool, it's about trees. Its new shoe, Tree, is made from FSC-certified eucalyptus trees woven into a lightweight sustainable fibre.